Energy pellet

ABSTRACT

A high-energy water-resistant pellet of at least 75% torrefied wood and the remainder a binder comprising from about 2% to about 20% by total weight of the pellet. The binder is a two-component system—a plasticizer, such as tall oil pitch, rosin, fatty acid, vegetable oils, animal oils and corn protein—preferably from 3% to about 20% of the binder, and lignin from about 80-97% of the binder. A durable, low dust white wood energy pellet is also provided using the same two-component binders.

CROSS-REFERENCE TO RELATED APPLICATIONS

The contents of Provisional Application Ser. No. 61/673,573 filed Jul. 19, 2012, on which the present application is based and benefit claimed under 35 U.S.C. § 119(e), is herein incorporated by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to energy pellets made from torrefied wood and whole-tree (white) wood and methods of making same. More particularly, this invention relates to energy pellets made from torrefied wood or white wood combined with lignin and a biomass-based plastizer to form a water-resistant and durable high-energy-density pellet.

(2) Description of Related Art

In recent years renewal energy sources have become more desirable and thus more important although the United States continues to be heavily reliant on the combustion of non-renewable fossil fuels to meet its energy needs. Additionally, energy from biomass mandates in the European Union has led to a rapidly growing export market for energy pellets. Exports of white-wood pellets to the EU from North America was 4.4 million metric tons in 2012 and is predicted to reach 25-70 million metric tons per year by 2020. To meet the growing energy needs biomass based materials have been pelletized to provide a dense, relatively high energy material.

One of the desirable biomass materials being developed is torrefied wood. Torrefied wood is wood that has been partially oxidized to drive off the water and the organic volatiles and a fraction of the cellulose, so that a “blackened” wood chip is the result. The torrefied wood chips are friable and can be easily formed into a high-energy, high-density pellet. When a pellet is made from southern yellow pine wood with its high resin content, it will remain as a pellet for days when immersed in water—indicating excellent water repellency. A pellet made from southern yellow pine needs no binder, since the pellets from that wood is exposed to the 40,000+ psi of the extruder and temperatures exceeding 100° C., the natural components flow to form a water repellant binder naturally. However, when a pellet is made from torrefied wood, most of the natural binders are burned off so that the resulting pellet has poor water repellency, and in a worst case, the pellets turn into a black “mud” upon exposure to water.

Another problem with torrefied wood and even with white wood pellets, especially those made from low-resin-containing biomass, is durability. The pellets themselves tend to be friable and create dust during mechanical processes needed to transport the pellets, such as loading and unloading rail cars and ships. As much as 5-7% loss of mass from dust has been reported for white-wood pellets. In addition to product loss, the dust creates hazards, from inhalation and the potential for dust explosions.

Torrefied wood has the strong advantage of being closer to coal in its burning capabilities than white wood. And the energy (bulk) density of torrefied wood is much greater than white wood so its shipping costs are much lower.

The high-energy, high-density and coal-like mechanical properties of torrefied wood pellets are desirable for large biomass burning facilities—like the utilities but the coal is normally transported in uncovered cars to and from ports and stored in uncovered piles at the combustion site, so water repellency is a key attribute. A single utility would have to spend $100 million or more to cover these pellet piles and rail cars, and few if any will make this capital expenditure, so water repellency is strongly desirable.

Currently wood pellets are burned, but the lower energy density and their pulverizing and burning characteristics limit their use as a fuel since they are so different from coal. Energy pellets made entirely of lignin has approximately the same energy content as coal, about 12,000 Btu/lb, which is about 50% higher energy per mass of low-moisture wood pellets having about 8,000 Btu/lb.

Within the wood matrix, lignin is a macromolecule is chemically bound to cellulose and hemicellulose. In pulping processes or enzymatic biorefineries, the lignin is separated from the cellulose and hemicellulose and its molecular weight is reduced (i.e., the lignin is “chopped up”). This isolated lignin at ambient conditions is a solid, but it can be melted at elevated temperature, with lower molecular weight lignin generally having a lower melt point. The problem is that isolated solidified lignin is very friable, and its melt point may not be ideal for the pelletizing system so that the lignin melts and flows within the pelletizer but solidifies upon exiting and cooling.

The lignin content in wood has recently been shown (Thomas Wilson, PhD Thesis, Penn State University, 2010) to have no effect on the durability of white wood pellets. In the Penn State study, a wide variety of wood with varying lignin contents was used; however, all that lignin was in the macromolecular form and chemically bound to cellulose. Contrary to the Penn State study, lignin with lower molecular weight that has been isolated from wood has better binding capability. And the macromolecular lignin used in the Penn State study may not have the required melt-point profile.

For a binder to work well in forming energy pellets, the binder must melt and flow under the extruder conditions of elevated temperature and pressure. As the pellets emerge from the extruder and cool, the binder must reform as a solid, coating and protecting the smaller elements—loose fibers of white wood or particles of torrefied wood.

Lignosulfonate is a byproduct of the sulfite pulping process. Lignosulfonates have demonstrated commercial utility as binders for animal foods. Lignosulfonates are a poor choice as energy pellet binders because: (1) lignosulfonates have high levels of organically-bound sulfur—as high as 10% by mass—that converts to sulfur dioxide upon combustion; and (2) lignosulfonates contain residual sugar from the pulping process. These sugars are hydrophilic and deleteriously affect the water resistance of energy pellets. The sulfonate groups on the lignin are also hydrophilic, exacerbating the water resistance problem.

SUMMARY OF THE INVENTION

There is provided a high-energy water-resistant pellet having at least 75% torrefied wood and the remainder being a two-component binder comprising from about 2% to about 25% by total weight of the pellet. The starting material is torrefied wood having a moisture content of below 15% water, with the equilibria water content being about 10-15% water. The binder is a two-component system—with a plastizer to adjust the melt point, such as tall oil pitch, rosin, fatty acids, animal oils, vegetable oils, or corn-oil proteins, preferably from 3% to about 20% of the binder, and lignin from about 80-97% of the binder.

There is also provided at method for making a high-energy water-resistant pellet from torrefied wood and a two component binder.

It is therefore the general object of the present invention to provide a high-energy, high density fuel pellet made from torrefied wood or white wood and a binder that is water repellent.

Another object of the present invention is to provide a high-energy, high-density water-repellant fuel pellet from torrefied wood or white wood that includes a two-component binder.

Yet another object of the present invention is to provide a process for producing a high-energy pellet.

Other objects, features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the preferred embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention now will be described more fully hereinafter in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather these embodiments are provided so that this disclosure will be through and complete and will fully convey the scope of the invention to those skilled in the art.

A preferred fuel pellet of the present invention is produced from torrefied wood having a moisture content of below 15% water, with the equilibria water content being about 10-15% water, that has been mixed with a binder and compressed by passing through an extruder, typically at a temperature at which the binder is in a molten state while the torrefied wood is compressed. Forming the pellets commonly involves hydraulic extrusion through a fuel pellet die. Of course, other suitable compression equipment known to those skilled in the art may be used. The temperature of the torrefied wood and binder in the extruder is above 100° C. but is not allowed to exceed 280° C. to avoid adverse exothermal torrefication conditions. Preferably the temperature is maintained between 100° C. and 165° C. Another embodiment of the present invention uses whole-tree (white) wood in place of torrefied wood. For use in this invention the white-wood is ground to a size that allows free-flow into the throat of the extruder.

The water-resistant high-energy pellet comprises at least 75% torrefied wood and preferably at least about 85% torrefied wood, and the remainder a binder comprising from about 2% to about 25%, preferably about 3% to about 15%, by total weight of the pellet. The binder comprises of a plasticizer, such as tall oil pitch, preferably from 5% to about 20% by weight of the binder, and lignin from about 80% to about 97% by weight of the binder. The optimal ratio of the lignin to plasticizer will depend upon the characteristics of the torrefied wood, the lignin, and the plasticizer that are contained within the pellet. Those choices typically are made with economics being the primary factor.

Lignin, a by-product of the pulp and paper industry, of sufficiently low T_(g) will flow and provide protective coverage within the pellet that is needed for water repellency but may not flow under the conditions of the pelletizer which operates at 105° C. and ≧40,000 psi. The individual characteristics of the lignin chosen may not be suitable without a plasticizer since its T_(g) may be too high which inhibits its flow at the temperature of the pelletizer. Adding a plasticizer that itself is water resistant can provide the requisite T_(g) allowing the binder to flow within the pelletizer yet solidify at ambient temperature. Again this ratio will be driven by performance and economics since lignin generally will be much less expensive than tall oil pitch once the multiple large-volume lignin production facilities come on-line from papermaking operations and enzymatic conversion of biomass that will have lignin as a byproduct stream.

The binders contemplated for use in this invention are totally natural organics, preferably tall oil pitch, fatty acids, rosin and the like. Tall oil pitch is the bottoms product from the first distillation column of a refinery that has tall oil as a feedstock and makes distilled rosins and fatty acids as products. The tall oil pitch may be used, or the “spent” tall oil pitch from which the valuable sterol fraction has been removed could be used.

The binders of this invention may also include vegetable or animal oils to plasticize the lignin while providing water repellency. Corn oil and corn protein isolated from ethanol processing provides a ready commercial source of natural plasticizers. Corn protein has demonstrated binding capability, and provides additional binding capability to the lignin.

Tall oil pitch is an excellent binder for fire logs, and TOP is currently commercially used for this application. The cost of TOP can be relatively high ($400-$600/ton). The “spent” tall oil pitch, which has the valuable sterol fraction removed is less expensive.

Torrefied wood pellets have a bulk density (40-45 lbs/ft³), about twice that of non-pelletized torrefied wood (16-20 lbs/ft³), so making pellets dramatically reduces volume and subsequent transportation costs. Torrefied wood pellets have an energy density about the same as coal, 12,000 Btu/lb. Size-reduction characteristics of wood pellets—how they break down under mechanical forces—are much different than that of coal, which is a problem when large fractions of wood pellets are fed into existing pulverizing equipment used by all coal-burning power-generation equipment. However, size-reduction characteristics of torrefied wood pellets are very similar to those of coal. Thus the mechanical handling characteristics and energy density of torrefied wood pellets are very similar to coal, making those pellets much more acceptable as a direct substitute for coal.

As noted, one of the critical characteristics of the high-energy-content torrefied wood pellets of this invention is that they are water repellant. Another important property is pellet strength and attrition resistance, since dust creates not only worker health and safety issues but also dust severely reduces the bulk density of the pellets so that a ship loaded with pellets crossing an ocean carries a lower mass loading causing the shipping rates per ton to be higher. This densification effect is counter-intuitive because the dust particles separate the individual pellets thereby decreasing the bulk density instead of partitioning entirely in the interstitial areas of the bulk pellets.

Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. 

1. A high-energy water-resistant pellet comprising: (a) at least 75% torrefied wood; and (b) the remainder of said pellet being a two-component binder comprising from about 2% to about 20% by total weight of the pellet wherein said binder is a two-component system comprising a plasticizer selected from the group consisting of tall oil pitch, fatty acids, rosin, vegetable oil, animal oils and corn protein and the other component is lignin.
 2. The high-energy water resistant pellet according to claim 1 wherein said high-energy water-resistant pellet comprises at least 85% torrefied wood and less than 10% by weight water.
 3. (canceled)
 4. (canceled)
 5. The high-energy water resistant pellet according to claim 4 wherein said plasticizer is from about 3% to about 20% of said binder.
 6. The high-energy water resistant pellet according to claim 1 wherein said lignin is from about 80% to about 97% of said binder.
 7. The high-energy water resistant pellet according to claim 1 wherein high-energy water resistant pellets have a bulk density 40-45 lbs/ft,
 8. A high-energy water-resistant pellet comprising: (a) at least 85% torrefied wood; and (b) the remainder of said pellet being a two-component binder comprising from about 2% to about 20% by total weight of the pellet, wherein one component of said binder is a plasticizer selected from the group consisting of tall oil pitch, fatty acids and rosin, vegetable oil, animal oils and corn protein and the other component is lignin in an amount from about 80% to about 97% of said binder.
 9. A high-energy water-resistant pellet comprising: (a) at least 75% white wood; and (b) the remainder of said pellet being a two-component binder comprising from about 2% to about 20% by total weight of the pellet wherein said binder is a two-component system comprising a plasticizer selected from the group consisting of tall oil pitch, fatty acids, rosin., vegetable oil, animal oils and corn protein and the other component is lignin.
 10. (canceled)
 11. (canceled)
 12. The high-energy water resistant pellet according to claim 11 wherein said plasticizer is from about 5% to about 20% of said binder.
 13. The high-energy water resistant pellet according to claim 11 wherein said lignin is from about 80% to about 97% of said binder.
 14. The high-energy water resistant pellet according to claim 11 wherein high-energy water resistant pellets have a bulk density 40-45 lbs/ft³.
 15. A high-energy water-resistant pellet comprising: (a) at least 85% white wood; and (b) the remainder of said pellet being to two-component binder comprising from about 2% to about 20% by total weight of the pellet, wherein one component of said binder is a plasticizer selected from the group consisting of tall oil pitch, fatty acids, rosin, vegetable oil, animal oils and corn protein and the other component is lignin in an amount from about 80% to about 97% of said binder.
 16. A process for making an energy pellet from torrefied wood or white wood comprising: (a) mixing at least 75% of said wood; and the remainder of said mixture being a two-component binder comprising wherein one component of said binder is a plasticizer selected from the group consisting of tall oil pitch, fatty acids, rosin, vegetable oil, animal oils and corn protein and the other component is lignin, said binder being from about 2% to about 20% by total weight of said mixture; and (b) compressing said mixture by extruding said mixture at a temperature at which said binder is in a molten state to form a pellet.
 17. The process for making an energy pellet from torrefied wood according to claim 16 wherein said mixture is extruded at a temperature from about 100° C. to about 280° C.
 18. The process for making an energy pellet from torrefied wood according to claim 16 wherein said mixture is extruded at a temperature from about 100° C. to about 165° C. 